The invention is directed to a power amplifier for the feed of an inductance with switched transistors that are arranged in a bridge circuit in the diagonal of which the inductance is connected. Every transistor has a freewheeling diode allocated to it such that, when the transistor is deactivated, the diode assumes the current conduction thereof. A plurality of transistors are connected in parallel in every bridge branch.
Such a power amplifier is disclosed, for example, by DE-A1-34 38 034, incorporated herein by reference. An arbitrary current path in both directions can thus be set by an appropriate drive of the transistors of the bridge circuit. Compared to a linear amplifier concept, considerable advantages with respect to space requirement, dissipated power, and thus costs, result due to the switched operation of the transistors. Bipolar transistors having high performance capability are available, so that high currents can also be switched without having to connect a great number of transistors in parallel. A compact structuring technique having a short line layout, and thus a low line inductance, is thus possible. However, it is particularly the switching losses in bipolar transistors having high current-carrying capability that are still considerable due to limited switching speed, so that one quickly arrives into power regions wherein a water cooling is required. A serious disadvantage in the employment of bipolar switching transistors is the limited controllability of the power amplifier. As a consequence of the retention time, bipolar transistors having high current-carrying ability have a switch-off delay of approximately 5 .mu.s. A noteworthy safety margin between the individual switching events must be observed because of this switch-off delay and because of the differences between bipolar transistor units. For two transistors which lie in series with respect to the operating voltage, two things are prevented: that they conduct simultaneously and that transverse currents arise. The necessary safety margin lies at approximately 10% of the switching cycle. In control-oriented terms, this safety margin acts as dead time, which is expressed as a deterioration of the obtainable curve shape, particularly given low currents.
Significantly higher switching speeds, and thus reduced switching losses, can be achieved with power MOSFET transistors. MOSFET transistors fundamentally have no retention time, so that the dead time can be noticeably reduced, and thus the controllability of an amplifier is improved. A disadvantage of the power MOSFET transistors compared to bipolar transistors is the lower current-carrying ability. When high currents are required from a power amplifier, many MOSFET transistors must be connected in parallel.
High demands made of power amplifiers occur, for example, in gradient amplifiers of nuclear magnetic resonance tomographs. Coil arrangements for generating linear magnetic field gradients must thus be supplied with current. For example, the following, typical demands made of such power amplifiers result:
(a) the currents must be capable of being exactly set in a range from 10 mA through 250 A;
(b) currents in two directions are required;
(c) a power curve shape prescribed by a drive must be reproduced as precisely as possible;
(d) the power amplifier must supply an output voltage that assures an adequate rate of the current rise in the gradient coil. A characteristic value for the output voltage, for example, is .+-.300 V;
(e) given an optimally high nominal current, the power amplifier must allow an optimally high "duty cycle"; and
(f) the power amplifier must be capable of being executed in three-dimensionally small fashion and without a need for liquid coolant, insofar as possible.
The problem of uniform current division into the transistors connected in parallel arises given the parallel connection of many transistors and a high switching speed. Particularly given switch-over events, there is the risk that--due, for example, to inductances--the current division is non-uniform, which can quickly lead to a destruction of transistors.